Extreme Hydroclimate Events Between Early-1100s and Mid-1200s Recorded in Deep Springs Lake, California

Previous studies reported megadroughts in southwestern North America during the Medieval Climate Anomaly and subsequent periods. However, previous local records challenge traditional tree-ring based indices. Through mineralogical analyses of Deep Springs Lake sediments in California, we examined carbonate mineral assemblages as novel hydroclimate proxies. Our investigation revealed a primary magnesite-rich layer (∼70% Ca-bearing magnesite, ∼30% dolomite) beneath a layer dominated by aragonite and calcite, implying a transition from an extreme drought to pluvial conditions. Since magnesite precipitates only at the highest Mg:Ca ratios, followed by dolomite and calcite/aragonite, we developed the quantitative Carbonate Dryness Index (CDI) by weighting these minerals. The CDI curve highlights a drought and a following pluvial with unparalleled magnitudes in the past millennium during 1090–1150 CE, aligning with nearby local climate proxies but not regional tree-ring records. Additionally, DSL sediments recorded ∼500-year wet/dry cycles driven by solar and El Niño-Southern Oscillation (ENSO) activities.